There's a Hormone That Burns Fat and Keeps It Off — and Scientists Have Proven It Works

Irisin, the flab-fighting hormone, is triggered by working up a sweat, according to new research.

As far as raw science is concerned, losing weight—that is to say, burning fat—is a fairly straightforward process.

As any gym rat will tell you, shaving away your love handles and belly fat all depends on running a "caloric deficit," which means burning more calories than you eat. Some back-of-the-envelope math: Because a pound of body fat roughly equates to 3,500 calories, operating at a 500-calorie deficit for an entire week should, theoretically, translate to being one pound lighter.

And for a long time, the mathematics of weight loss started and ended with calories. Countless dieters and bodybuilders governed every meal and every treadmill session by a bizarre calculus of calories, slaving away to hit those ironclad calorie numbers and make those weight loss goals happen. But new research suggests your body might have another way to translate exercise into burning fat—and it has nothing to do with calories. Instead, it's all about hormones.

Hitting the gym stimulates your body to release a hormone that helps transform your white fat cells, which just lounge around storing energy, into brown fat cells, which actively burn energy, according new research from the University of Florida. And that's not all: The hormone, called irisin, also discourages your body from creating new fat.

In the Florida study, which was published in the American Journal of Physiology: Endocrinology and Metabolism, lead researcher Li-Jun Yang, M.D., collected fat cells from 28 women who had undergone breast reduction surgery. When the researchers added irisin to some of the samples, they discovered that the fat tissue had five times more UCP1, which is crucial to "burning" fat. Furthermore, the irisin eliminated as much as 60% of the fat cells in the treated samples, the researchers found.

“We used human fat tissue cultures to prove that irisin has a positive effect by turning white fat into brown fat and that it increases the body’s fat-burning ability,” said Yang in a press release from UF Health. They theorize that the irisin could prevent stem cells from becoming fat cells, thereby limiting the amount of new fat that can be created.

To be clear: Your body's irisin surge isn't totally responsible for fat burn. Calorie deficits still matter. It's also hard to say exactly how exercise promotes irisin in the body, especially because the study was done in tissue samples and not live patients. But this new research does suggest that your body has more than one way of controlling its fat content. It also suggests that working out consistently might (emphasis on might) hormonally condition your body to keep fat off—so that even when you're not actively sweating off pizza night on the elliptical, your body could be better tuned to burn off excess energy instead of immediately shuttling it into your fat cells.

Ultimately, though, if you want an irisin fix, you'll still need to do what you've been doing: hit the gym. Irisin might help obese and diabetic patients in the future, but until then, the squat rack will be waiting for you.

“Instead of waiting for a miracle drug, you can help yourself by changing your lifestyle. Exercise produces more irisin, which has many beneficial effects including fat reduction, stronger bones and better cardiovascular health,” Yang said.